Earlier quoted context omitted.
I went from sea level to 11k feet in the same day many times before. I would say the altitude effect is there but not as much as you might expect. A little quicker to be out of breath a little longer to recover it. Not sure what it is like at higher elevations or greater daily altitude delta.
I guess fitness makes a difference from what these other comments are saying. My ex-wife and I lived in Long Beach (which is obviously sea level) when we were in ROTC and pretty regularly took day trips out to San Gogornio and walked to the summit in about five hours, which is ~11,500ft. Not once did that have any noticeable effect, but we were both pretty serious runners back in the day trying to become Army officer…
Not really. Altitude sickness seems quite random in who it effects worst. I trekked to the top of Mera Peak (~21,000 ft) many years ago. 3 of the fittest people in our party got altitude sickness and didn't make it to the peak.